The Exotic Wave in 190 GeV p p at COMPASS Tobias Schl - PowerPoint PPT Presentation

The Exotic η ′ π − Wave in 190 GeV π − p → η ′ π − p at COMPASS Tobias Schl¨ uter for the COMPASS collaboration Ludwig-Maximilians-Universit¨ at M¨ unchen June 16, 2011

πη ′ in diffractive scattering Possible quantum numbers for the πη ′ system: S -wave P -wave D -wave F -wave G -wave · · · L J PC 0 ++ 1 − + 2 ++ 3 − + 4 ++ · · · Hence: P -wave resonant → exotic meson. This system has been studied by the following experiments: experiment beam momentum reaction year published VES 37 GeV / c π − N → η ′ π − N 1993, 2005 E852 18 GeV / c π − p → η ′ π − p 2001 They all see a very strong P -wave.

πη ′ in diffractive scattering Possible quantum numbers for the πη ′ system: S -wave P -wave D -wave F -wave G -wave · · · L J PC 0 ++ 1 − + 2 ++ 3 − + 4 ++ · · · Hence: P -wave resonant → exotic meson. This system has been studied by the following experiments: experiment beam momentum reaction year published VES 37 GeV / c π − N → η ′ π − N 1993, 2005 E852 18 GeV / c π − p → η ′ π − p 2001 COMPASS 190 GeV / c π − p → η ′ π − p 2012 (?) They all see a very strong P -wave.

Previous πη ′ results – VES Results from VES (Be target, 37 GeV ): ◮ VES sees the a 2 (1320) (peak in D + -wave) ◮ VES says: “there may be an a 2 (1700)” explaining the broad structure in the D + -wave ◮ VES says: “there may be an exotic π 1 (1600)” Note the jump in the relative P + − D + phase near 2 GeV

Previous πη ′ results – BNL E852 Results from BNL E852 (proton target, 18 GeV ): ◮ they see the a 2 (1320) (peak in D + -wave) ◮ they add a G + -wave to the fit, gives: a 4 (2040) ◮ they explain the broad D + -wave with an a 2 (1700) and the P + -wave with an exotic resonance π 1 (1600) ◮ they find an unusual t -slope Note the various jumps at 2 GeV

Remarks on PWA formalism For a given mass, two-body states in the reflectivity basis ( ǫ, ℓ, m ) have the form ( θ, φ Gottfried-Jackson angles) � sin( m φ ) ǫ = +1 ( m > 0) A ǫ lm ∝ Y m ℓ ( θ, 0) cos( m φ ) ǫ = − 1 ( m ≥ 0) The observed intensity is then proportional to 2 2 � � � � � � � � � � I ( θ, φ ) = T +1 ,ℓ m A +1 ,ℓ m ( θ, φ ) + T − 1 ,ℓ m A − 1 ,ℓ m ( θ, φ ) � � � � � � � � � � � � lm ℓ m where the production amplitudes T ǫℓ m were introduced. Important observations: only negative reflectivity (= unnatural exchange) contributes to intensity at φ = 0; all positive reflectivity waves with m = 1 have the same φ dependency. Negative reflectivity compatible with zero in VES, BNL, COMPASS analyses.

The COMPASS spectrometer ◮ fixed target experiment at CERN’s SPS accelerator ◮ variety of beams available (pos/neg muon, pos/neg hadron) ◮ variety of targets (polarized targets, LH2, solid state) ◮ diverse physics program ◮ 2008, 2009 : hadron beam runs with various targets In this talk: 2008 data, negative pion beam at 191 GeV , LH2 target

Data selection Final state selected: exclusive 3 tracks, 2 photons × 3 × 3 10 10 entries / 0.5 MeV entries [MeV] (2 entries per event) COMPASS 2008 COMPASS 2008 100 π - → π - π - π γ γ π - → π - π - π η p + p p + p 12 w/o acceptance correction 80 10 8 60 6 40 preliminary preliminary 4 20 2 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 1.5 2 2.5 3 3.5 4 4.5 γ γ π - π η m( ) m( + ) [GeV] ◮ select exclusive events with 3 tracks + recoil proton, 2 good ECAL clusters ◮ select η → 2 γ (left) ◮ select η ′ → π − π + η (right)

Data selection Final state selected: exclusive 3 tracks, 2 photons × 3 × 3 10 10 entries / 0.5 MeV entries [MeV] (2 entries per event) COMPASS 2008 COMPASS 2008 100 π - → π - π - π γ γ π - → π - π - π η p + p p + p 12 w/o acceptance correction 80 10 8 60 6 40 preliminary preliminary 4 20 2 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 1.5 2 2.5 3 3.5 4 4.5 γ γ π - π η + m( ) m( ) [GeV] Result: entries / 20 MeV COMPASS 2008 π → π η π - π γ γ - - + 600 p ’( )p ◮ 18 000 events with w/o acceptance correction 500 m ( η ′ π ) < 2 GeV / c 2 , 35 000 400 total 300 ◮ mass reach beyond 2 GeV / c 2 preliminary 200 ◮ additionally, about 3 000 events 100 0 in πη ′ , η → 3 π channel 1.5 2 2.5 3 3.5 4 4.5 5 π η m( ’) [GeV]

First look at the data: t slopes We find indication for a continuous transition between different production mechanisms, fitting the t distribution in several areas mass bin fit with A exp( − B | t | ) fit with A | t | exp( − B | t | ) m < 1 . 5 5.5 8.2 1 . 5 < m < 1 . 9 5.1 7.5 1 . 9 < m < 2 . 2 4.8 7.1 2 . 2 < m < 3 4.6 6.9 (BNL fitted with a simple exponential between 0 . 25 < | t | < 1 . 0 GeV / c 2 , they found B = 2 . 93 / GeV 2 ) We find: higher mass → broader slope and: clear contradicition with BNL

Input to the PWA Like previous analyses, we used all waves with ℓ ≤ 2, m ≤ 1 and additionally the ǫ = +1, ℓ = 4, m = 1. I.e.: ǫ = +1 P + D + G + ǫ = − 1 S 0 P 0 P − D 0 D − As expected from Pomeron (i.e., natural) exchange, the negative reflectivity waves turn out compatible with zero (below 2 GeV ). Ambiguities are bounded by the size of the negative waves, i.e. they are not a problem.

PWA results – P + and G + waves 8000 ) [deg] 20 2 events / 40 MeV/c COMPASS 2008 COMPASS 2008 π → π η π → π η - - - - p ’p 0 p ’p 7000 PC -+ P (J = 1 ) intensity -20 phase D - P + + / P + 6000 + -40 + arg(D 5000 -60 -80 4000 Intensity P + -100 Phase D + − P + 3000 preliminary -120 preliminary 2000 -140 1000 -160 -180 0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 m [GeV] m [GeV] 2 COMPASS 2008 events / 40 MeV/c π - → π - η p ’p 6000 PC ++ D (J = 2 ) intensity + 5000 Intensity D + 4000 3000 preliminary 2000 1000 0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 m [GeV] Clear phase-motion from a 2 (1320), jump in phase near 2 GeV , slow phase-motion in range of P + -wave intensity peak.

PWA results – can the G + -wave clarify the picture? 8000 20 2 ) [deg] ) [deg] 800 events / 40 MeV/c COMPASS 2008 COMPASS 2008 COMPASS 2008 π - → π η - π - → π - η π - → π - η p ’p 0 p ’p p ’p 7000 PC -+ P (J = 1 ) intensity phase D - P 700 phase G - P + -20 + + / P + + / P + + 6000 -40 + + arg(D arg(G 600 5000 -60 Int. P + 4000 -80 500 -100 3000 preliminary -120 preliminary 400 preliminary ∆Φ( D + − P + ) ∆Φ( G + − P + ) 2000 -140 300 -160 1000 -180 0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 m [GeV] m [GeV] m [GeV] 2 ) [deg] events / 40 MeV/c COMPASS 2008 400 COMPASS 2008 π - → π - η π - → π - η p ’p p ’p 6000 PC 350 D (J = 2 ++ ) intensity phase G - D + + / D + + 5000 300 + Int. D + arg(G 4000 250 200 3000 150 preliminary preliminary 2000 ∆Φ( G + − D + ) 100 1000 50 0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 m [GeV] m [GeV] 1600 2 events / 40 MeV/c COMPASS 2008 π - → π - η p ’p 1400 PC G (J = 4 ++ ) intensity Int. G + + 1200 1000 800 600 preliminary 400 200 0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 m [GeV] Clear phase-motion in G + -wave relative to D + wave, compatible with a 4 (2040). Again: jump at 2 GeV in phase relative to P + wave. But: unlike between P + and G + no rapid phase jump between D + and G + waves at 2 GeV

Transition between different production processes? 1 GJ 40 θ 0.8 cos 35 0.6 COMPASS 2008 π → π η 30 - - p ’p 0.4 w/o acceptance correction 25 0.2 0 20 -0.2 15 preliminary -0.4 10 -0.6 5 -0.8 -1 0 1.5 2 2.5 3 3.5 4 4.5 5 m [GeV] Depicted: cos θ GJ of the η ′ in the π − η ′ GJ restframe vs. m ( πη ′ ). Low masses show P and D wave interference, a 4 near 2 GeV / c 2 , above that strong forward/backward peaking indicative of central production. Question: How does the forward/backward peaking at high masses affect the interpretation at low masses?

Comparison to ηπ We also selected the ηπ − final state along the same lines. No PWA yet, for comparison, here’s the same plot as on the previous slide, but for the πη : 1 GJ θ 0.8 cos 0.6 COMPASS 2008 π → π η - - p p 0.4 w/o acceptance correction 0.2 0 preliminary -0.2 -0.4 -0.6 -0.8 -1 1 1.5 2 2.5 3 3.5 4 4.5 m [GeV] Depicted: cos θ GJ of the η in the π − η GJ restframe vs. m ( πη ). Dominated by a 2 (1320), structures due to a 4 (2040) visible, again forward/backward peaking at high masses.